Carat is the term used to describe the weight of any gemstone, including diamonds. Although the definition of a carat has changed over time, since 1913 the international standard has been 200 milligrams, or 1/5 of a gram. Often, jewelers describe carats in 1/8 increments.

In jewelry pieces with more than one diamond, the carats may be described in terms of total carat weight (T.W.). This is the combined total weight of all the stones in the piece.

Because round brilliant cuts follow exact standards, you can make a good estimate of the carat weight of the stone based on the stone's diameter. The following chart compares the relative sizes of stones and describes how much a round brilliant diamond of a certain size is likely to weigh. This chart is for educational purposes and represents a guideline for diameter and carat weights. It is not representative of other cuts or shapes. It is also not applicable to colored gemstones which have a different density from diamonds.